1. Field of the Invention
The present invention relates to improved packaging techniques for optical communication systems and, more particularly, to techniques for providing a cost-effective package suitable for either transmitter or receiver applications.
2. Description of the Prior Art
There exist many different arrangements for packaging optical transmitters or receivers. For example, the optical device (e.g., laser, LED, or PIN photodiode) may be encased in an optical assembly with appropriate lensing and an optical fiber attached to the assembly. The electronics required for operating the optical device (a modulation circuit for a transmitter or a detection circuit for a receiver) are separately assembled (on a printed wiring board, for example) and connected to the housed optical device by conventional electrical leads. Alternatively, the electronics may be fully enclosed in a separate housing, with only the leads exposed for connection to the optical device. An advantage of using separate housings is that many different electronic circuits may be connected to the same optical device. For example, the electronics for a data link application may operate with various types of logic signals (e.g., ECL or TTL) without the need for altering the package housing the optical device. Additionally, if either the electrical or optical portion of a transmitter or receiver were to experience a failure, the working portion may be disconnected and re-used with another arrangement. A major disadvantage of separate packaging is the overall size of the transmitter or receiver. With two separate packages, or an optical package mounted on a printed wiring board, the system requires a rather large space. Further, such a system is susceptible to electrical noise introduced by the necessarily long lead lengths of the connections between the electronics and the optical device. This electrical noise factor becomes a significant concern at bit rates exceeding approximately 10 Mb/s. Also, long leads may limit the maximum bit rate of both the transmitter and receiver, due to parasitic lead inductance (limits transmitter) and parasitic capacitances (limits receiver).
These and other concerns have led to the development of package designs which provide for the electronics and optical device to be housed in the same unit. In particular, there exist a number of such unitary arrangements particularly suited for data link applications which utilize relatively low bit rates (&lt;=50 Mb/s) and operate over relatively short distances (&lt;=3 km). Most of these applications use an LED as the transmitting device and a PIN photodiode as the receiving device. Many of these unitary packages are relatively expensive, as a result of using a hybrid integrated circuit (HIC) arrangement for the electronics, with the optical subassembly attached to the HIC. Additionally, the optical subassemblies associated with these packages are often formed of machined metallic components, adding to the cost of the system. Further, these subassemblies have been known to experience alignment difficulties. There also exist fabrication problems in mating the various piece parts (i.e., outer housing, optical subassembly and HIC). These unitary packages are also known to generate a significant amount of heat, where thermal management of optical data link transmitters and receivers has been a significant problem. Lastly, in many instances, the packaging processes for a transmitter and receiver are very distinct, leading to manufacturing problems and increasing the overall expense of the packaged system.
Thus, a need remains in the prior art for improving the packaging techniques utilized to form optical transmitters and receivers, particularly where the necessary electronics are to be housed with the optical device.